1. Field of the Invention
This invention relates to the field of power transmission for use in mechanized irrigation equipment, and more particularly to the wheel drive gearboxes that drive the tires supporting and propelling the irrigation system.
2. Description of the Related Art
Center pivot and linear irrigation systems are utilized to irrigate large sections of land. Typically, such systems are capable of watering a quarter section of land, i.e., 160 acres, or more. A center pivot irrigation system is an approximately one-quarter mile mechanized pipe that rotates around a fixed pivot point. When activated, the irrigation pipe rotates around the pivot, emitting liquids over the area. The length of time required for a 360 degree rotation of the irrigation pipe typically range from 12 hours up to several days. Linear irrigation systems are comprised of similar irrigation structure, but move linearly across sections of land to be irrigated, typically by wire guidance.
In conventional irrigation systems, the irrigation pipe spans are 120 to 180 foot lengths supported by a wheeled tower at each intersecting span. Sprinklers are located at spaced intervals either along the length of the span or along a water conduit parallel to the span. The wheels of each tower are normally positioned perpendicular to the span such that the system moves laterally. When constructed, wheels tracks are established to follow a prescribed path, either circular for center pivot systems or linear for linear systems. Each tower is typically provided with one or more drive motors operable to move the tower. Generally, power from the motors is transferred to the wheels via a wheel drive gearbox connected to each wheel. Most conventional systems either utilize an electric gearmotor mechanically connected between the pair of wheel drive gearboxes, or hydraulic or electric motors directly coupled to the wheel drive gearboxes.
Agricultural wheel drive gearboxes are generally comprised of a gearbox housing in which is mounted a power input shaft having a worm gear defined thereon. A spur gear is axially attached to a power output shaft which is transversely mounted in the gearbox relative to the power input shaft such that the teeth of the spur gear engage the worm gear. A lubricant, such as heavy industrial grease or oil is disposed within the gearbox. Seals are provided at the points where the power shafts protrude from the gearbox. These prior art seals commonly consist of one or more o-rings or similar rubber sealing elements disposed around the shaft in sealing contact therewith. The standard operating pressure for such a seal is about 10-15 psi. Typically, these wheel drive gearboxes are not designed to withstand the full range of hazardous environmental conditions to which irrigation systems are likely to be exposed, such as moisture, dirt, insects and temperature extremes. Particularly vulnerable in these gearboxes are the oil seals located at the power shaft exit points. Such seals are utilized to maintain an oil bath for the internal gears and are therefore pressurized. As such, maintaining the integrity of the seal and the internal pressure within the gearboxes under the extreme working conditions of irrigation systems becomes crucial. These oil seals are most commonly lipped oil seals formed of nitrile rubber.
Generally, in order to maintain internal oil bath or lubricant pressure within the gearbox, lipped oil seals rely upon either a fixed polymer or nitrile lip having a face or contact surface compressed against the steel surface of a rotating shaft. Prior art irrigation drive line gearbox seals might also be mechanically compressed, i.e. loaded, to enhance sealing pressure by such means as a bellows or spring mechanism to urge the sealing elements into contact with one another. Without this xe2x80x9cpreloadingxe2x80x9d, the gearbox will likely experience leakage at the seal face, especially as internal gearbox pressures rise. In this regard, prior art irrigation drive line gearboxes are likely to require an expansion chamber or vent plug to allow oil to completely submerge the worm gear while not pushing the seals out of the gearbox, i.e., creating excess pressure at the sealing interface. In another iteration of these prior art seals, an expanded gear housing is provided above the worm gear, thereby increasing the air cavity to allow some pressure compensation (thermal expansion) and preventing a blow-out of the seal. One drawback to the above-described nitrile rubber lip seals is that such seals fatigue much more quickly under the high internal pressures which may be generated in these types of gearboxes.
Specifically, these oil seals become wear points along the contact line between the lip face and the rotating metal drive shaft. Generally, all prior art polymer or nitrile seal lips harden and erode with age such that the intersecting metal surfaces are subject to environmentally induced corrosion and wear from the rotating seal. This creates particular vulnerability to the wheel drive gearboxes. As the seals wear, they eventually lose elasticity and sealing contact, resulting in a loss of internal gearbox pressure. Once internal pressure is lost, the gearbox lubricant leaks from the internal bearings and gears of the wheel drive, contaminating the internal gearbox environment, which may result in rapid total failure of the wheel drive gearbox.
As mentioned above, wheel drives exposed to agricultural environments are subject to many different hazards such as moisture, corrosive liquids including liquidized manure, the full range of soil and mineral conditions, insects and temperature extremes which could range from continental polar to equatorial. All of these hazards can damage the integrity of the irrigation system wheeldrives, and in particular, the lipped or cartridge oil seals used to maintain pressure within the gearbox. This is especially true for these types of prior art seals since their incorporation into a irrigation drive line gearbox leaves them at least partially exposed to the external environment.
More particularly as to these prior art wheel drive gearboxes, they are unable to maintain extreme case pressure arising from the varied operating environments without creating a gearcase vacuum pressure during cooling. These variances in case pressure are a contributing factor to premature failure in lipped and cartridge type seals. Further, the gearcase vacuum created when cooling may draw hygroscopic moisture into the gearcase, further contaminating the lubricant and resulting in accelerated gear and bearing wear.
Typically, irrigation systems are placed into service upon completion of construction and initial inspection. Thereafter, they are infrequently examined or maintained. As a result, when a wheel drive gearbox loses lubrication, it is unlikely to be detected. Loss of lubricant may result in seizure of the gears within the gearbox and when the gearbox has seized, the entire irrigation drive train and watering system may be subject to automated shutdown. One solution is to more frequently check and maintain the gear drives to the irrigation system. However, it is often difficult to determine when a seal will ultimately fail by visual inspection. Therefore increased inspection may not reasonably assure detection. Furthermore, in that such irrigation systems are otherwise automated and located in remote areas, such a solution is time consuming and expensive. In addition, since the system must be shut down to safely inspect the gear drive seals, there is a down time associated with such a procedure.
It is therefore an object of the invention to provide the means and apparatus to hermetically seal the gearcase from the environment.
It is also an object of the invention to maintain a high-pressure gear drive.
It is a further object of the invention to provide a gear drive that requires much less maintenance than prior gear drive.
It is a further object of the invention to provide a gear drive that is less likely to experience oil seal failure than prior art gear drives.
These and other objects are achieved through the wheel drive gearbox rotating oil seal system of the present invention in which the gearbox is hermetically sealed, such that the gearcase and components within can withstand the full range of extreme environmental and operating conditions without lubricant loss or failure during the stated design life. The rotating oil seal system provides a mechanical face seal comprised of compressed and sintered silicon and carbide elements whose surfaces create a sealing interface under a preloaded tension established during assembly. This ceramic-carbide face seal is functionally constructed within the gearbox housing, eliminating direct contact from solar radiation, especially ultraviolet radiation. The mechanical elements are made from inert silicates immune to metallic oxidation and wear. The preload of the sealing elements allow internal or external pressures to exceed 3 atmospheres without fatiguing or failing the components.
The above described invention is advantageous over the prior art because it is functionally impervious to environmental and application related failure modes, with the only wear surface being the interface between the silicon and carbide rotating elements, obviating the need for constant attention as is necessary with the prior art devices. In addition, the above described invention protects the gearbox""s internal lubricant from a vacuum condition intake of hygroscopic moisture and other environmental hazards such as fertilizers, pesticides, fungicides, and liquidized effluent from the temperature extremes that could result in lubricant loss or failure. Further, the system can be easily retrofit the existing gear drives in the market.